Searching for and Characterizing GeV Gamma-ray Binaries from Multiwavelength Observations

Transcription

1 Searching for and Characterizing GeV Gamma-ray Binaries from Multiwavelength Observations Robin H.D. Corbet 1, C.C. Cheung 2, M.J. Coe 3, J.B. Coley 1, P. Edwards 4, V. McBride 5, M.V. McSwain 6, J. Stevens 4 1: University of Maryland, Baltimore County/NASA Goddard Space Flight Center 2: Naval Research Laboratory 3: University of Southampton 4: CSIRO Astronomy and Space Science 5: University of Cape Town 6: Lehigh University

2 How to Make a Gamma-ray Binary? Two ingredients needed: Power source. Non-thermal mechanism. e.g. Fermi acceleration at shocks + inverse Compton scattering. The conventional mechanisms are: Accreting microquasar (stellar mass black hole) with relativistic jets. Pulsar interacting with the wind of a hot (O or B type) companion. Pulsar and stellar winds collide and form shocks. 2

3 High-Mass X-ray Binary/Gamma-ray Binary Connection? X-ray binaries may go through a gamma-ray binary phase early in their lives. A newly born neutron star is expected to be rapidly rotating and highly magnetized. Relativistic pulsar wind interacts with companion's wind and produces gamma rays until neutron star has spun down (e.g. Dubus 2006). Meurs & van den Heuvel (1989) predicted ~30 such systems in the Galaxy in this brief phase. Pulsar wind pressure dominates for: 3

4 HMXBs Born as Gamma-ray Binaries? HMXBs containing neutron stars may begin as gamma-ray binaries with furiously rotating neutron stars before spinning down. 4

5 The Fermi LAT Fermi was launched on June 11, The primary instrument is the LAT: 100 MeV (or lower) to 300 GeV (and higher). The LAT has several advantages over previous detectors: - Instrument performance: Improved effective area, field of view, angular resolution compared to EGRET. - Observation mode: the LAT has operated in sky survey mode most of the time. The entire sky is observed every ~3 hours. Can study binaries on a wide range of timescales. 5

6 Few! Gamma-ray Binaries are Known The Fermi LAT initially detected: Cygnus X-3: transient microquasar, Wolf-Rayet (+ black hole?). 4.8 hr orbital period. LS I : Be star, suspected pulsar companion day orbital period. PSR B : Be star, definite 48ms pulsar companion. 3.4 year orbital period. LS 5039: O6.5V((f)) star, suspected pulsar companion. 3.9 day orbital period. Either the mechanisms that make gamma-ray binaries occur infrequently, or else more systems (as predicted) remain to be discovered! These were all suspected gamma-ray binaries - even before Fermi. (Cyg X-1 & HESS J not strongly detected with LAT. Cyg X-1 detected with AGILE. Bodaghee report marginal Fermi detection) 6

7 The Hunt for New Binaries Known gamma-ray binaries show modulation on their orbital periods. Hope to find new binaries from the detection of periodic variability. Even with the improved sensitivity of the LAT, count rates are still low. A bright source may only give ~20 photons/day. Need to have highest possible signal-to-noise light curves and to make sensitive period searches. 7

8 Optimizing Light Curves There are two basic ways to make LAT light curves: Maximum likelihood fitting. Aperture photometry. Likelihood fitting is slow, and is difficult/impossible if few/zero photons are present in a time bin. Aperture photometry is not optimal. Ignores source photons outside the aperture, includes background inside the aperture. Problem compounded by strong LAT PSF energy dependence. Instead, use a weighted photon/infinite aperture technique. Sum the probabilities that each photon came from source of interest. Can give a significant increase of Signal/Noise. 8

9 Optimizing Power Spectra To search for periodic modulation, use power spectra. We want ability to search for short orbital periods, like Cyg X-3's 4.8 hour period. Short time bins are needed (e.g. < 1ks). Shorter than the LAT sky survey period of ~3 hours. This gives big variations in exposure. Use exposure weighting of each data point's contribution to the power spectrum. 9

10 Weighting Power Spectrum Benefit Exposure-weighting of power spectra is crucial for short (600s) time bins. Benefits reduced for longer time bins (e.g. 1 day) with less exposure variability. 10

11 Probability Weighted Photometry: Bright Source For very bright sources there is not necessarily any benefit using probability weighted photometry rather than regular aperture photometry. 11

12 Probability Weighted Photometry: Fainter Source For fainter sources, probability weighted photon photometry can give large gain in signal-to-noise level. 12

13 History: First Large Search and Discovery of First! New GeV Source (Corbet, Cheung, Kerr+, 2012) The first Fermi LAT catalog (1FGL) contained 1,451 sources. Made light curves for all sources: 3 degree radius aperture. 600 s time bins (barycenter corrected). 100 MeV to 200 GeV. Calculated exposure-weighted power spectra for all sources: 0.08 days to length of light curve (2 years at that time). Easily detected LS I and LS 5039, but not Cyg X-3. (To detect Cyg X-3 must only use data from active states.) And a candidate for a new binary... 13

14 The Discovery of the Binary 1FGL J Probability of peak at 16.6 days arising by chance is < Second (and possibly higher) harmonics of this period are also seen. 14

15 Flux/Spectral variability on 16.6 day period Both gamma-ray flux and spectrum of 1FGL J are modulated on the 16.6 day period. Source is harder when bright. 1FGL J

16 Large X-ray variability (Swift XRT) - Different colors (top panel) show X-ray data from different 16.6 day cycles. - Flare-like behavior with peak coinciding with gamma-ray maximum. - X-ray modulation also has a quasi-sinusoidal component with peak at phase ~0.4 Additional Swift observations (An et al. 2013) do not show flare at phase 0(!), but do still show quasi-sinusoidal component. 16

17 Stability of 1FGL J Additional Swift observations (An+ 2013) show that X-ray modulation is not as stable as initially thought: 17

18 Optical Spectrum of 1FGL J H, He I/II lines indicate early spectral type. He II 4686Å absorption main sequence. He II/I ratio ~O6. NIII emission O6V((f)). Spectral type is almost identical to LS SAAO 1.9m telescope Interstellar absorption lines E(B-V) ~1.25. V ~12.6 (ASAS Catalogue) Distance ~5 kpc (± 2kpc) 18

19 Variable Radio Counterpart (ATCA Radio data) The radio flux appears to be modulated on the orbital period. But, no increase at phase 0. Radio flux may be following sine wave component of X-ray flux. 19

20 TeV Counterpart to 1FGL J1018.6? The H.E.S.S. team (2012) reported discovery of HESS J Positionally coincident with SNR G and 1FGL J (diffuse extension towards PSR J ). TeV emission is seen (at least sometimes) from LS 5039 and LS I , Is this the TeV counterpart of 1FGL J1018.6?? Temporal variability in the TeV source would confirm association. Bordas+ (2013) report first evidence of flux variability 20 R. Corbet et al.

21 1FGL J Overview 1FGL J was the first new gamma-ray binary found with Fermi. X-ray, optical, and radio counterparts were identified. We don't definitely know what is driving the gamma-ray emission. J may contain a rapidly rotating pulsar interacting with the wind of an O star. But other explanations might be possible. e.g. magnetar model proposed for LS I (Torres+ 2012). The multi-wavelength observations were key to demonstrating that this is a gamma-ray binary. 21

22 Classification via Multi-Wavelength Two facets of multi-wavelength approach: If a gamma-ray source has a stellar counterpart, then it is extremely likely to be a gamma ray binary. If modulation at same period as seen in the gamma-ray source is found at another wavelength, the identification is definite. Hierarchical approach: Obtain X-ray image of Fermi error box: Swift can reveal brightest sources, Chandra and XMM provide deeper images. For all X-ray sources detected: Obtain optical spectra, optical photometry, search for periodicity Search for (variable) radio counterpart with ATCA etc. 22

23 Continued Growth of Binary Signals in Known Systems In the three persistent GeV binaries, the signal strength in the power spectrum has continued to increase. LS I s modulation is more variable. We hope/expect that there are other binaries that are slowly climbing in Signal/Noise. 23

24 Contrasts in Long-Term Variability of Gamma-ray Binaries (i) Wide range in the long-term variability of gamma-ray binaries. Cygnus X-3. Microquasar, Wolf-Rayet primary + black hole (??). 4.8 hr period. Transient: emits gamma-rays for brief periods of time during soft X-ray states. Power spectrum of total light curve reveals only long-term variability.! To clearly see orbital period in power spectrum must select only active! states. 24

25 Long-Term Variability of Gamma-ray Binaries (ii) LS 5039: 3.9 day period, O6.5V primary. Gamma-ray and X-ray emission appear very stable on multiyear timescales (e.g. Hadasch+ 2012). Corbet+; auto-analysis power spectrum 25

26 Long-Term Variability of Gamma-ray Binaries (ii) LS I Be star system with 26.5 day orbital period. Proposed 1667 day superorbital radio and optical period (similar to superorbital periods in Be X-ray binaries). Long-term GeV variability in flux and binary modulation (Hadasch+ 2012). Ackermann+ (2013) propose GeV properties modulated at day (4.6 year) period. (But, not yet covered 2 cycles!) Superorbital Phase s -1 ] -2 ph cm -6 Flux [ Average Power Unit Time [MJD] Ackermann+: black = flux, blue = orbital modulation 0 Corbet+; auto-analysis power spectrum 26

27 Long-Term Variability of Gamma-ray Binaries (iii) 1FGL J O6V primary (like LS 5039) with 16.5 day period. As noted earlier, X-ray modulation initially seemed very stable, but An found changes in X-ray modulation. There is no strong long-term modulation in gamma-ray flux. We are investigating long-term multiwavelength variability: LAT, ATCA, Swift, etc. (Coley+ 2014). Corbet+; auto-analysis power spectrum 27

28 The Missing GeV Counterpart to HESS J Swift XRT HESS J first binary identified from VHE emission. ~320 day period found in X-rays with Swift (Bongiorno+ 2011, Aliu+ 2014) Optical counterpart MWC 148 is B0pe, radial velocity measurements give e ~0.8 (Casares+ 2012) No detection with LAT at any binary phase (Caliandro+ 2013). erg cm -2 s -1 ] -12 Flux ( kev) [x MJD MJD MJD MJD phase Fermi LAT TS residual map! after removal of sources. 28

29 Fermi Modified Observing Strategy Since start of regular observations, Fermi mostly operated in sky-survey mode. Entire sky observed every two orbits (~3 hours). Since end of 2013 new observing strategy in place: intersperse pointed observations of Galactic center with sky-survey. Benefits: Study pulsars near Galactic center Peri-center passage of G2 cloud - coordinated with other missions Better statistics for dark matter search at Galactic center Maintain all-sky coverage Increases exposure of binaries near Galactic center (HMXB population known) 29

30 Binary Break from Galactic Center Focus PSR B has Be star primary and 47.8ms pulsar in 3.4 year orbit. Next periastron passage ~2014 May 4. Galactic center observations reduced coverage around B1259. For the next ~3 months Fermi returns to regular sky survey observations. Better coverage of B1259 periastron passage (also lower temperature improves battery performance). Red lines mark periastron passage.! Note that previous LAT maximum occurred 30 days after periastron! (Chernyakova+ 2014) Corbet+ 30

31 The Colliding Wind Binary η Carinae η Car is exceptional - contains one of the most massive and most luminous stars in the Galaxy. Great Eruption in mid-19th century: became 2nd brightest star in sky for ~decade (now not even top 100) Thought to be binary with highly eccentric (e ~0.9) 5.5 year period Primary is luminous blue variable M > 100 M, L ~ L Secondary O or Wolf-Rayet star Colliding winds form shocked gas X-ray emission modulated on 5.5 year period Corcoran

32 η Car as a Gamma-ray source? Spatial coincidence between AGILE (Tavani+ 2009) and LAT (Abdo+ 2010) gamma-ray sources and η Car: 2FGL J Apparent variability of J (Reitberger+ 2012) 2 Modified Julian Day 54,800 55,000 55,200 55,400 55, Flux [10-7 photons cm -2 s -1 ] blue line = periastron months Not a gamma-ray binary as it doesn t contain a compact object, but may be gamma-ray emitting binary! Now have one orbital cycle and can start to look for binary modulation to prove η Car association: next periastron is late July/early August

33 Fundamental Revisions to LAT Data Extraction Extraction of gamma-ray information from the LAT is complicated. Gamma rays create electron-positron pairs in the LAT. Electrons & positrons create tracks, and release their energy in the LAT tracker and calorimeter. The tracker and calorimeter information must then be used to reconstruct the energy and direction of the gamma ray. Must distinguish between low gamma-ray rate, and higher rate charged particle background. 33

34 LAT Data Releases Versions of algorithms/software are called Passes. Pass 6 was used at launch, Pass 7 reprocessed (P7REP) is the current version. P7REP uses improved calibration, includes correction for small time-dependent change in energy scale. Relative light yield PRELIMINARY Time since launch [years] 34 Bregeon et al. (2013) The LAT team is also working on more fundamental changes to processing with Pass 8...

35 The Promise of Fermi LAT Pass 8 Pass 8 is radical revision. Potential improvements include: Significant reduction in background contamination. Increased effective area. Improved point spread function. Atwood et al. (2013) 4th Fermi Symposium : Monterey, CA : 28 Oct-2 Nov 2012 If these improvements are realized, then it will provide an enormous boost for photon-starved binary searches! 35

36 Summary Very few gamma-ray binaries known, even though a population of pre-high-mass X-ray Binaries is predicted. We have developed techniques to increase signal-to-noise of LAT light curves and power spectra (and to search for nonsinusoidal modulation). One completely new system (1FGL J ) was found from 2 years' worth of data. There are hints of more systems, and we are using a multiwavelength approach to classify these sources. 3rd LAT catalog (3FGL) is being finalized: new sources and better source parameters. Pass 8 LAT processing, and continued observations, promise improved sensitivity (particularly Galactic center). 36